The present invention relates to an optical communications system for performing two-way optical communications between, in particular, digital audio apparatuses and between information-processing apparatuses, and also relates to an optical transmit-receive module and an optical transmit-receive coupler to be used for such an optical communications system.
FIGS. 31A and 31B show conceptual diagrams of a technique disclosed in Japanese Patent Laid-Open Publication No. HEI 6-140106 (Applicant: Sharp Kabushiki Kaisha). This technique is to provide an optical transmit-receive module with a hole that allows a plurality of types of plugs to be commonly inserted, thereby allowing the use of not only an optical fiber plug but also the existing electric plugs. Further, a plurality of electrical connection terminals are provided in the hole, thereby allowing an identical optical transmit-receive module to be used for not only transmission and reception of an optical signal but also transmission and reception of an electrical signal. FIG. 31A shows a case where a miniature single-headed type electric plug (for analog electric use) is inserted in the hole, while FIG. 31B shows the case where an optical fiber plug is inserted in the hole.
In FIG. 31A are shown a miniature single-headed type electric plug 71, terminals 72 for electrical connection, and a light-emitting section 73. An electrical signal applied via the electrical connection use terminal section 72 is transmitted to the electric plug 71 and transmitted to the other end by way of an electric cable connected to the electric plug 71. When the optical transmit-receive module operates in electric use, the light emitting section 73 is in an unoperated state.
In FIG. 31B are shown a light-emitting section 73, an optical fiber section 74 and an optical fiber plug 75. A transmission light emitted from a semiconductor light-emitting chip 74 mounted on the light-emitting section 73 is collected by a lens 75 formed on a surface of an encapsulating resin and made incident on an optical fiber 76. Reference numeral 77 denotes an integrated circuit chip for driving the semiconductor light-emitting chip 74. The optical signal transmitted through the optical fiber 76 is received by an optical transmit-receive module provided on the other end. The optical transmit-receive module on the other end is almost similar to the optical transmit-receive module shown in FIG. 31B, wherein a light-receiving device is used in place of the semiconductor light-emitting chip, and an integrated circuit chip for processing an optical reception signal is used in place of the driving integrated circuit chip. When the optical transmit-receive module operates in optical use, the electrical connection terminals 72 are in an unoperated state.
Connection between the various plugs and the apparatus terminals will be described next. FIG. 32 is an explanatory view for explaining a state in which three identification terminals V1, V2 and V3 are in contact with the connector section, wherein the type of the currently inserted connector section is identified by using potential differences with respect to a reference voltage Vref and a ground GND.
FIG. 34 shows a table indicating correspondence between types of the plugs (connectors) and outputs of the terminals, wherein a great potential difference is indicated by "H" and a small potential difference by "L". The plug is of analog electric use when the terminal outputs V1, V2 and V3 are L, L and L, respectively, the plug is of digital electric use when the outputs are L, L and H, the plug is of digital optical fiber plug use when the outputs are L, H and H, and no plug is inserted when the outputs are H. H and H.
Either the miniature single-headed type electric plug or the optical fiber plug is used as a connector. The miniature single-headed type electric plug includes a plug for analog electrical signal and a plug for digital electrical signal. The optical fiber plug includes a plug for digital optical signal, as shown in FIG. 33. Referring to FIG. 33, in the case of the plug for analog electrical signal, examples of the input side and the output side are LINE IN MIC and LINE OUT HEADPHONE, respectively. In the case of the plug for digital electrical signal, examples of the input side and the output side are a digital input (coaxial input) and a digital output (coaxial output), respectively. In the case of the plug for digital optical signal, examples of the input side and the output side are an optical digital input and an optical digital output, respectively.
In the case of the aforementioned prior art example, unidirectional optical communications have been performed by means of one optical fiber. For the two-way communications, two optical fibers have been necessary. In this case, because of the use of two optical fibers, there has been a problem that installing the fibers and adjusting the optical coupling between the light-receiving devices and light-emitting devices are more difficult than in the case of using a single optical fiber. Furthermore, in order to perform the two-way communications by means of one optical fiber in optical communications using only light, there have been needed an optical branching type light guide and a special light-receiving section and a light-emitting section appropriate for the light guide. In an optical transmit-receive module employing a bifurcated light guide, the bifurcated light guide is designed so as to divide one beam of light into two beams of light or to combine two beams of light into one beam of light, and this has led to a problem that an efficiency in transmitting and receiving light, important for the two-way communications, is degraded.
In Japanese Patent Laid-Open Publication No. HEI 8-130507, an optical transmit-receive module for performing both fiber transmission and optical space transmission was proposed by Sharp Kabushiki Kaisha. This is shown in FIG. 35. In this optical transmit-receive module, an optical signal from a light-emitting section 197 is transmitted to a light-receiving section 199 via an optical guide 198, a space 100 and another optical guide 198 and converted into an electrical signal. The transmission between the optical guide 198 and the optical guide 198 is a spatial transmission by divergent light or parallel light.
In this optical transmit-receive module, the optical guide sections require a space for escaping from front of the light-emitting device or the light-receiving device, and this has led to a problem that the size of the optical module increases.
Furthermore, the fact that the optical incidence diameter is the tip diameter of each optical guide section has led to a problem that the signal light cannot be picked up much, resulting in a reduction in the transmission distance.
There has conventionally been a further problem that dust and dirt tend to adhere to the connection between a light guide and an optical fiber inside an optical transmit-receive module and the connection between an optical fiber and an optical fiber inside an optical transmit-receive coupler.
There has been a further problem that reflected return light increases due to the dust and dirt adhering to a surface of the connection portions between the light guide and the optical fiber inside the optical transmit-receive module and the connection between the optical fiber and the optical fiber inside the optical transmit-receive coupler.